Citrate esters

ABSTRACT

Citrate esters are formed utilizing organic titanates as a catalyst allowing excess alcohol to be removed. Four citrate esters have been found which provide advantageous plasticizing properties to PVC compositions which include superior toxicity test results and superior soapy water extraction test results. The four citrate esters are: acetyltri-n-hexyl citrate, n-butyryltri-n-hexyl citrate, acetyltri-n-(hexyl/octyl/decyl) citrate, and acetyltri-n-(octyl/decyl) citrate. Articles formed from the PVC plasticized mixtures are extremely useful in the medical or health care field as they demonstrate a low order of toxicity.

This is a divisional application of application Ser. No. 07/117,080 filed 04 Nov. 1987 which was a continuation of Ser. No. 06/865,874 filed 21 May 1986 now U.S. Pat. No. 4,711,922 which was a continuation of pending patent application Ser. No. 06/735,149 filed 17 May 1985, now abandoned which as a continuation-in-part of application Ser. No. 06/691,583 filed 11 June 1984, also now abandoned.

BACKGROUND OF THE INVENTION

1. Field Of The Invention

Citrate esters are useful as plasticizers for polyvinyl chloride (PVC) resins as certain of these esters provide a low order of toxicity when compared to phthalate esters which have been conventionally used. Other advantages have been noted using certain citrate esters as plasticizers in PVC compositions and articles, including improved resistance to soapy water extraction and low temperature and transport properties.

The preparation of the citrate esters has been found to be significantly enhanced by the utilization of certain organic titanate catalysts which allow the excess alcohol to be removed after the esterification step.

2. Description Of The Prior Art And Objectives Of The Invention

Citrate esters commercially produced using citric acid have long been available and have been used as plasticizers for PVC resins. However, the performance of articles produced from the PVC resin compositions whether utilizing citrate esters known to date or conventional phthalate plasticizers have had many inherent disadvantages. For example, medical-grade PVC compositions are used to form blood bags, tubing and a variety of health-related articles and in recent years toxicity has been a major concern for manufacturers of such articles. Recent reports have identified di-2-ethylhexyl phthalate (DEHP) or (DOP) and di-2-ethyl-hexyl adipate (DEHA) as hepatocarcinogens in rodents. While certain of the phthalates have excellent plasticizing qualities, their suspected carcinogenic nature renders them doubtful candidates for future medical-grade uses. As an alternative, known citric acid esters such as acetyltri-n-butyl and tri-n-butyl citrate were tried as PVC plasticizers in medical-grade applications but it was determined that these compounds were not entirely satisfactory due to their high soapy water extraction percentages and would therefore not be useful in many medical area applications. Also, it has been found that new production techniques had to be devised for the newer citric acid esters which were determined to have suitable toxicity and physical characteristics when used as PVC plasticizers.

It is therefore an objective of the present invention to provide PVC plasticizers which provide superior toxicity test results in biological studies.

It is also an objective of the present invention to provide plasticizers for PVC compositions which can be processed without difficulty using conventional extrusion, calendering, or plastisol techniques.

It is yet another objective of the present invention to provide new citric acid esters namely: acetyltri-n-hexyl citrate, n-butyryltri-n-hexyl citrate, acetyltri-n-(hexyl/octyl/decyl) citrate, and acetyltri-n-(octyl/decyl) citrate which can be used as plasticizers having desirable physical characteristics when imparted into PVC compositions.

It is still another objective of the present invention to provide PVC compositions and formed articles therefrom having superior results in toxicology studies concerning dermal toxicity, oral toxicity and genetic assays.

It is also an objective of the present invention to provide a new process for the low temperature manufacture of the four new citric acid esters utilizing organic titanates to provide economical and efficient production methods.

Others objectives and advantages of the present invention will be demonstrated to those skilled in the art as set forth in detail below.

SUMMARY OF THE INVENTION

Citrate esters of the formula: ##STR1## where R₁, R₂, and R₃ =CH₃ to C₁₈ H₃₇,

R₄ =CH₃ to C₇ H₁₅

and more specifically acetyltri-n-hexyl citrate, n-butyryltri-n-hexyl citrate, acetyltri-n-(hexyl/octyl/decyl) citrate, and acetyltri-n-(octyl/decyl) citrate are produced utilizing an organic titanate catalyst and such esters have been found useful as medical-grade plasticizers in PVC compositions. The plasticizers have a low order of toxicity and inpart to PVC the proper balance of physical properties needed in health care and medical-grade uses. The production steps for the citric acid esters include low temperature esterification at 140° C. or below, removal of any excess alcohol and thereafter, alkoxylation. Conventional neutralization and finishing steps are then carried out. The alkoxylation step is carried out at a temperature less than approximately 110° C.

A PVC resin can be combined with one of the above-mentioned citric acid esters, along with suitable stabilizers and lubricants, to form a plasticized PVC which can be extruded, calendered or otherwise processed into suitable articles of manufacture including blood bags, tubing and other products. Articles so made have a low order of toxicity and provide superior extraction properties, particularly in soapy water extraction tests. The soapy water extraction test is a standard test, the results of which closely resemble the results obtained with body fluids such as human blood.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The four preferred forms of the citrate esters are as follows: ##STR2##

The preferred method of manufacture of the above-identified citrate esters comprises low temperature esterification below 150° C. and preferably at a temperature range of from 125° C. to 130° C. of the proper alcohol (such as n-hexyl alcohol for acetyltri-n-hexyl citrate) with citric acid in the presence of the organic titanate, tetra-n-butyl titanate, removal of any excess n-hexyl alcohol, and then alkoxylation of the esters produced with an acid anhydride. At esterification temperatures above 150° C. citrates undergo rapid degradation resulting in numerous products of decomposition. At temperatures somewhat below 150° C. the major decomposition product is an aconitate ester. The alkoxylation takes place at a temperature of below approximately 110° C. Tetra-n-butyl titanate is preferred since the ester interchange which takes place between the titanate alkyl groups and citrate alkyl groups does not result in the introduction of alkyl groups not normally present in the citrate esters.

The preferred PVC composition comprises blending and milling a medium molecular weight PVC resin with one of the above citrate esters on a two to one ratio, resin to plasticizer, along with stabilizers, lubricants and extenders as required. Articles manufactured from the preferred PVC compositions include blood bags, tubing and other articles for the medical and health care fields.

DETAILED DESCRIPTION OF THE INVENTION

Certain citrate esters, namely acetyltri-n-hexyl citrate, n-butyryltri-n-hexyl citrate, acetyltri-n-(hexyl/octyl/decyl) citrate and acetyltri-n-(octyl/decyl) citrate have been found to be particularly useful in medical applications when compounded with PVC resins through conventional plastisol, calendering or extrusion techniques. Such plasticized PVC exhibits good clarity, good low temperature properties, low volatility and low extractability into various media. Also, a low order of acute toxicity has been shown and complete compatibility with medium molecular weight PVC resins make the four named esters unique and valuable. Studies have shown that the four citrate esters are not toxic substances, primary skin irritants or ocular irritants to unrinsed eyes and oral administration has produced no signs of systemic toxicity and has shown no mortality in fasted mice or rats. Genetic toxicology assays for detecting mutagenic activity at the gene or chromosomal level have shown that these esters do not induce gene mutation in either microbial cells or in mammalian cells in vitro or chromosomal mutation in vivo or in vitro. Stuides have also shown that under in vivo conditions, these citrate esters hydrolyze rapidly and completely in concentrations at expected realistic levels of human exposure.

Preparation of the citrate esters are as follows:

EXAMPLE 1 Preparation of acetyltri-n-hexyl citrate

330 lbs. of n-hexyl alcohol, 180 lbs. of citric acid and 1.54 lbs. of tetra-n-butyl titanate and 15 gallons of heptane are charged to a vessel equipped with agitator, thermometer, vapor column, condenser and a decanter set to allow removal of water formed during the reaction while refluxing heptane. The esterification is effected at 140° C. to maintain the aconitate (THA) level below 0.5% for general production. As shown in FIG. 1 the aconitate level can be kept well below the 0.2% range by longer reaction times at lower temperatures with the optimum time, and aconitate levels reached by temperatures of from approximately 125° C. to 130° C. As shown, at a temperature of approximately 130° C. a unique result is achieved in that the aconitate formation levels out to provide a citrate ester having excellent purity. During esterification water is periodically removed from the decanter in order to maintain proper temperature and reaction rates. The esterification is continued until the esterification mixture tests 0.5% maximum acidity calculated as citric acid although lower temperature esterification and acidity percentage may be used for higher purity products as mentioned above. Next, the vessel is cooled to 120° C. and any water is removed from the separator and any heptane therein is also removed for future use. The reflux line of the vessel is closed and pressure on the system is reduced slowly. The kettle is heated to 130°-140° C. and steam is introduced to help remove any residual alcohol. This vacuum steam stripping is continued until alcohol cannot be detected by conventional laboratory tests. When no more alcohol can be found, the steam is discontinued and the temperature is reduced to 100° C. and the vacuum is broken with nitrogen gas.

Next, 0.4 lb. concentrated sulfuric acid (H₂ SO₄) is charged into the vessel after which it is sealed and approximately 107 lbs. of acetic anhydride (in a determined molar amount) are added at a slow rate so that the temperature does not exceed 110° C. When all the anhydride has been added, agitation of the mix continues for approximately one hour until the acetylation reaction has been completed.

Next, a full vacuum is put on the system and enough heat is added for distillation to proceed at a suitable rate. This step continues until acetic acid is shown to be 5% or less by conventional lab tests whereupon the mixture is cooled to 75° C. for neutralization.

The remaining steps of neutralization, bleaching, washing, etc. are carried out as in conventional esterification processes.

EXAMPLE 2 Preparation of n-butyryltri-n-hexyl citrate

The vessel used in example 1 is again charged with 330 lbs. of n-hexyl alcohol, 180 lbs. of citric acid and 1.54 lbs. of tetra-n-butyl titanate. Esterification is carried out as in example 1 as is the heptane-alcohol strip. Butyrylization is thereafter done with the addition of 0.4 lbs. of concentrated sulfuric acid and 166 lbs. of n-butyryic anhydride as shown above in the acetylation process. The butyric acid may be removed as shown above or by neutralization.

Examples 1 and 2 produce esters with the following characteristics:

    ______________________________________                                         ANALYTICAL DATA                                                                             Acetyltri-n-hexyl                                                                            n-Butyryltri-n-hexyl                                Property     Citrate       Citrate                                             ______________________________________                                         Purity wt %  99            99                                                  Color APHA   50 max.       50 max.                                             Neut. No. mg KOH/g                                                                          0.2 max.      0.2 max.                                            Moisture K.F.                                                                               0.25 max.     0.25 max.                                           S.G. @ 25/25° C.                                                                     1.0045-1.0055 0.991-0.995                                         R.I. @ 25/25° C.                                                                     1.445-1.447   1.444-1.448                                         Viscosity @ 25° C. cps                                                               25-35         25-35                                               Odor @ 25° C.                                                                        Little or none                                                                               Little or none                                      ______________________________________                                    

    ______________________________________                                         Heat Stability                                                                 (2 Hrs. @ 150° C.)                                                      ______________________________________                                         Color APHA        50-60    50-60                                               Neut. No. mg KOH/g                                                                               0.2 max. 0.2 max.                                            Odor @ 25° C.                                                                             Mild     Mild                                                ______________________________________                                    

It has been determined that a citrate ester yield can be achieved of 99+% purity with a minimum of aconitate formation and unacetylated esters by lowering the esterification temperatures to 130° C. or below with a preferable temperature range of 125° C. to 130° C. Table A and FIG. 1 demonstrate the percentage of tri-n-hexyl aconitate (THA) formed during the production of acetyltri-n-hexyl citrate whereby the reaction is terminated at approximately 0.2% acidity, as citric acid. As shown in Table A and FIG. 1 below, the aconitate levels range from approximately 0.14 to 0.19 with a reaction time of from 25 to 19 hours at temperatures of from 125° C. to 130° C. It has been determined that by lowering the temperature from 140° C. to 130° C. an additional reaction time of only 90 minutes is required with the aconitate level dropping from 0.35 to 0.19%, a decrease of approximately 45%. As shown, the aconitate level can be tremendously decreased by lowering the temperature approximately 10 degrees from 140° C. to 130° C. without substantially increasing the reaction time based on 0.2% acidity (citric acid) as the reaction completion indicator. As shown in FIG. 1 a stabilization of the aconitate formation occurs during esterification at a critical temperature of approximately 130° C. providing a technique for the manufacture of high purity esters having low aconitate levels. Lower aconitate percentages and other impurities provide the high quality plasticizer needed for medical-grade products.

                  TABLE A                                                          ______________________________________                                         Reaction Time and Aconitate                                                    Formation Rates at Various Temperatures                                        Esterification                                                                             Reaction   Final THA  % Acidity As                                 Temperature (° C.)                                                                  Time (Hrs.)                                                                               Content (%)                                                                               Citric Acid                                  ______________________________________                                         120         24 1/4     0.07       0.17                                         125         25         0.14       0.17                                         130         19         0.19       0.17                                         140         17 1/2     0.41       0.16                                         150         13         0.59       0.19                                         ______________________________________                                    

It is believed that acids such as citric acid with low pK values exhibit a synergistic effect with titanate catalysts at low temperatures in the 150° C. or lower range. Phthalic acid which has a high pK value will not undergo esterification with the titanate catalysts at these low temperatures.

Also, other organic titanate catalysts can be used to produce the four (4) esters of this invention such as tetrakis-2-ethylhexyl titanate although superior results have been demonstrated using tetra-n-butyl titanate.

    ______________________________________                                         PREPARATION AND TESTING OF PVC COMPOSITIONS                                    FORMULATION         PARTS BY WEIGHT                                            ______________________________________                                         Resin (Medium Molecular Weight                                                                     100                                                        PVC)                                                                           Plasticizer         50                                                         Stabilizer (Calcium/Zinc)                                                                          2.5                                                        Lubricant (stearic acid)                                                                           0.25                                                       ______________________________________                                    

The above formulation was blended and milled for 5-10 minutes at 325° to 340° F. The milled stock was pressed (3 min. at 340°-360° F. and 32,000 psi) to 40- and 70-mil sheets, and aged for 48 hours at room temperature for evaluation. All tests were made with samples cut from 70-mil pressed stock except for extraction tests which were obtained on 40-mil samples. The performance data was obtained by accepted ASTM methods with modifications as detailed below in Table B.

    ______________________________________                                         Tensile Strength                                                                             Determined with Instron TT, 1100                                 Ultimate Elongation                                                                          series (2 in./min.) using a                                      Modulus (100% elonga-                                                                        dumbbell-shaped specimen. Test                                   tion) (ASTM D638)                                                                            carried out at 70° ±° F.                        Hardness      Determined with Shore Durometer A                                (ASTM D676)   (10 sec.) at 75° ± 5° F.                        Torsional Flex (T.sub.4                                                                      Determined with Torsion Flex                                     and T.sub.f)                                                                   Tester of Clash and Berg design.                                               (ASTM D1043)  T.sub.4 is the temperature at which                                            the Modulus of Rigidity is 10,000                                              psi; T.sub.f is the temperature at                                             which the Modulus of Rigidity is                                               100,000 psi.                                                     Brittle Point Determined by impact method using                                (ASTM D746)   Scott Tester, Model E.                                           Volatile Loss (A/C)                                                                          Determined on specimens 2 inches in                              (ASTM D1203)  diameter heated in activated carbon                                            at 70° C. for 24 hours. Results                                         are expressed as percent of plasticizer                                        lost.                                                            Water extraction (Tap)                                                                       Determined on specimens 2 inches in                              Soapy Water Extraction                                                                       diameter suspended in appropriate                                (1% Ivory Flakes)                                                                            liquid at 60° C. for 24 hours.                            Oil Extraction                                                                               Results are expressed as percent                                 (ASTM NO. 3)  of plasticizer lost.                                             Migration Loss (silica)                                                                      Determined on specimens 2 inches in                                            diameter heated in silica (100 mesh),                                          at 70° C. for 24 hours. Results                                         are expressed as percent of plasticizer                                        lost.                                                            Volatile Loss (air)                                                                          Determined by Oven Method (24 hr. at                                           100° C.) on specimens 2 inches in                                       diameter. Results are expressed as                                             percent of plasticizer lost.                                     ______________________________________                                    

                                      TABLE B                                      __________________________________________________________________________     (PLASTICIZER PERFORMANCE DATA)                                                 PLASTICIZER     DEHP                                                                               DEHA                                                                               #1  #2  #3  #4  #5                                     __________________________________________________________________________     HARDNESS,                                                                      Durometer A, 10 Sec.                                                                           79  78  78  81  81  87  87                                     TENSILE, psi    2748                                                                               1797                                                                               2862                                                                               2978                                                                               2924                                                                               2743                                                                               2789                                   ULTIMATE                                                                       ELONGATION, %   395 414 400 390 427 364 374                                    100% MODULUS, psi                                                                              1368                                                                               1092                                                                               1348                                                                               1574                                                                               1362                                                                               1656                                                                               1704                                   T.sub.4 (10,000 psi),                                                          °C.      -8.4                                                                               -30.8                                                                              -7.6                                                                               -9.1                                                                               -11.9                                                                              -6.9                                                                               -4.0                                   T.sub.f (100,000 psi),                                                         °C.      -38.8                                                                              -66.5                                                                              -35.6                                                                              -41.6                                                                              -48.7                                                                              -53.1                                                                              -59.7                                  BRITTLE POINT, °C.                                                                      -24.5                                                                              -56.5                                                                              -18.5                                                                              -26.0                                                                              -33.5                                                                              -36.8                                                                              -37.8                                  VOLATILE LOSS, (air), %                                                                        4.8 7.1 12.1                                                                               2.6 1.7 .3  .1                                     VOLATILE LOSS, (A/C), %                                                                        3.4 7.6 7.0 1.7 1.4 2.8 4.5                                    WATER EXTRACTION, %                                                                            .7  1.5 1.2 1.9 1.7 1.5 3.3                                    SOAPY WATER                                                                    EXTRACTION, %   2.7 11.0                                                                               9.5 5.4 2.2 3.4 2.4                                    OIL EXTRACTION, %                                                                              11.4                                                                               34.7                                                                               10.9                                                                               13.8                                                                               15.7                                                                               15.2                                                                               19.3                                   SILICA GEL MIGRATION, %                                                                        12.2                                                                               23.0                                                                               17.0                                                                               4.4 3.6 4.8 7.4                                    __________________________________________________________________________      #1  acetyltrin-butyl citrate                                                   #2  acetyltrin-hexyl citrate                                                   #3  nbutyryltri-n-hexyl citrate                                                #4  acetyltrin-(hexyl/octyl/decyl) citrate                               

The plasticizer performance data in Table C demonstrates the results of tests with citric esters/expoxidized soybean oil (ESO) blends. ESO is commonly used in conjunction with DEHP at levels in the range of 1-5% based on DEHP as an aid in stabilization. The ratio of 2.5/97.5 ESO/citrate was used as a base point in the studies. Test results on this combination are shown in column 1. A significant improvement in properties, particularly soapy water extraction is noted.

                                      TABLE C                                      __________________________________________________________________________     (PLASTICIZER PERFORMANCE DATA)                                                 PLASTICIZER     2.5 ESO                                                                             20 ESO                                                                              40 ESO                                                                              40 ESO                                                                              40 ESO                                     PERCENTAGES:    97.5 #2                                                                             80 #2                                                                               60 #2                                                                               60 #3                                                                               60 #5                                      __________________________________________________________________________     HARDNESS,                                                                      Durometer A, 10 Sec.                                                                           81   80   80   81   85                                         TENSILE, psi    2907 3010 3079 3165 3097                                       ULTIMATE                                                                       ELONGATION, %   422  424  420  428  395                                        100% MODULUS, psi                                                                              1415 1429 1491 1514 1779                                       T.sub.4 (10,000 psi)                                                           °C.      -9.5 -7.8 -7.7 -8.2 -5.4                                       T.sub.4 (100,000 psi)                                                          °C.      -41.8                                                                               -41.3                                                                               -39.3                                                                               -41.8                                                                               -50.3                                      BRITTLE POINT, °C.                                                                      -26.5                                                                               -25.5                                                                               -20.5                                                                               -24.5                                                                               -26.5                                      VOLATILE LOSS, (Air), %                                                                        2.4  2.1  1.5  .8   .5                                         VOLATILE LOSS, (A/C), %                                                                        1.3  1.6  1.4  .9   1.1                                        WATER EXTRACTION, %                                                                            1.3  .9   .6   .8   1.0                                        SOAPY WATER                                                                    EXTRACTION, %   2.9  2.9  6.4  4.8  3.8                                        OIL EXTRACTION, %                                                                              13.0 11.6 10.1 10.0 12.9                                       SILICA GEL MIGRATION, %                                                                        5.7  5.3  4.7  4.0  2.5                                        __________________________________________________________________________      ESO -- Ester/epoxidized Soybean Oil                                            #2  acetyltrin-hexyl citrate                                                   # 3  nbutyryltri-n-hexyl citrate                                               #5  acetyltrin-(octyl/decyl) citrate                                     

Since ESO is less expensive than citrates, a reduction in plasticizer cost results if ESO can be substituted for part of the citrates. Results of tests with higher ESO/citrate ratios as shown in columns 2-5 of Table C and a significant improvement in properties up to and perhaps beyond the ratio of 20/80 ESO/citrate ratio as shown.

Various other PVC compositions can be formulated and the examples and illustrations shown herein are for illustrative purposes and are not intended to limit the scope of the invention. 

I claim:
 1. A citrate ester selected from the group consisting of acetyltri-n-hexyl citrate and n-butyryltri-n-hexyl citrate, said ester having an aconitate level of less than 0.20% when the esterification mixture from which said citrate is produced tests about 0.6% maximum acidity calculated as citric acid, said ester having heat stability characteristics, after heating at 150° C. for two hours, of a color not greater than 50-60 APHA and a mild odor at 25° C.
 2. A citrate ester selected from the group consisting of acetyltri-n-hexyl citrate and n-butyryltri-n-hexyl citrate, said ester having an aconitate level of not greater than 0.18% when the esterification mixture from which said citrate is produced tests about 0.9% maximum acidity calculated as citric acid, said citrate ester having heat stability characteristics, after heating at 150° C. for two hours, of a color not greater than 50-60 APHA and a mild odor at 25° C.
 3. The ester of claim 2 wherein said aconitate level is not greater than 0.15%.
 4. The ester of claim 2 wherein said aconitate level is not greater than 0.10%.
 5. A citrate ester selected from the group consisting of acetyltri-n-hexyl citrate and n-butyryltri-n-hexyl citrate, said ester having an aconitate level of less than 0.20% when the esterification mixture from which said citrate ester is produced tests in the range from about 0.6% to 0.2% acidity calculated as citric acid, said citrate ester having heat stability characteristics, after heating at 150° C. for two hours, of a color not greater than 50-60 APHA and a mild odor at 25° C.
 6. A citrate ester selected from the group consisting of acetyltri-n-hexyl citrate and n-butyryltri-n-hexyl citrate, said ester having an aconitate level of not greater than 0.15% when the esterification mixture from which said citrate ester is produced tests in the range from about 0.9% to 0.45% acidity calculated as citric acid, said citrate ester having heat stability characteristics, after heating at 150° C. for two hours, of a color not greater than 50-60 APHA and a mild odor at 25° C.
 7. A citrate ester selected from the group consisting of acetyltri-n-hexyl citrate and n-butyryltri-n-hexyl citrate, said ester having an aconitate level of not greater than 0.10% when the esterification mixture from whch said citrate ester is produced tests in the range from about 0.9% to 0.35% acidity calculated as citric acid, said citrate ester having heat stability characteristics, after heating at 150° C. for two hours, of a color not greater than 50-60 APHA and a mild odor at 25° C.
 8. The ester of claim 1 wherein said ester is acetyltri-n-hexyl citrate.
 9. The ester of claim 1 wherein said ester is n-butyryltri-n-hexyl citrate.
 10. The ester of claim 2 wherein said ester is n-butyryltri-n-hexyl citrate.
 11. The ester of claim 2 wherein said ester is acetyltri-n-hexyl citrate.
 12. A citrate ester selected from the group consisting of acetyltri-n-hexyl citrate and n-butyryltri-n-hexyl citrate, said ester having an aconitate level of less than 0.20% when the esterification mixture from which said citrate is produced tests about 0.6% maximum acidity calculated as citric acid wherein said ester has a heat stability characteristics, after heating at 150° C. for two hours, of a mild odor at 25° C.
 13. A citrate ester selected from the group consisting of acetyltri-n-hexyl citrate and n-butyryltri-n-hexyl citrate, wherein said ester has heat stability characteristics, after heating at 150° C. for two hours, of a color not greater than 50-60 APHA and a mild odor at 25° C.
 14. The ester of claim 13 wherein said ester is acetyltri-n-hexyl citrate.
 15. The ester of claim 13 wherein said ester is n-butyryltri-n-hexyl citrate.
 16. A citrate ester selected from the group consisting of acetyltri-n-hexyl citrate and n-butyryltri-n-hexyl citrate, wherein said ester has a heat stability characteristics, after heating at 150° C. for two hours, of a mild odor at 25° C.
 17. The ester of claim 16 wherein said ester is n-butyryltri-n-hexyl citrate.
 18. The ester of claim 16 wherein said ester is acetyltri-n-hexyl citrate. 